1. Field of the Invention
The present invention is in the field of silicon nitride synthesis. More particularly the present invention is in the field of methods and systems for low temperature synthesis of silicon nitride from silica, carbon and nitrogen gas using a catalyzed carbothermal reaction.
2. Review of Relevant Technology
There has been an increasing interest in the production and use of silicon nitride (Si.sub.3 N.sub.4) and related compounds, such as silicon oxynitride (Si.sub.2 N.sub.2 O). Silicon nitride is a highly chemically resistant material that can have a variety of uses. It can be used as a high temperature, chemically resistant ceramic material. It can also be used as a protective coating.
There are a variety of different ways to manufacture silicon nitride. One method disclosed in U.S. Pat. No. 4,387,079 to Kasai et al. involves heat treating a nitrogen-containing silane, such as tetra-amide-monosilane or silicon imide, with ammonia at a temperature above 400.degree. C. for a period of at least two hours to obtain silicon nitride. Preliminary, the nitrogen-containing silane is prepared by continuously reacting gaseous silicon tetra-chloride with gaseous ammonia at a temperature of -30.degree. C. to 70.degree. C. However, the drawback of the Kasai et al. process is that the reagents and methods disclosed therein are expensive and involve materials which can be harmful if handled improperly.
Another synthetic route for making silicon nitride is disclosed in U.S. Pat. No. 4,859,443 to Marosi. In Marosi, silicon nitride powders are prepared in a gas-phase reaction by reacting silicon tetrachloride with ammonia at above 500.degree. C. in a fluidized bed of silicon nitride particles. This method requires the use of a fluidized bed of silicon nitride and only results in the formation of silicon nitride deposits on the pre-existing silicon nitride particles within the fluidized bed.
U.S. Pat. No. 5,662,875 to Bachelard et al. discloses a process in which silica, carbon and a seed crystal of silicon nitride are reacted, in a nitrogen countercurrent, in the presence of a volatile metal selected from the group consisting of beryllium, magnesium, calcium, strontium, germanium, tin, titanium, hafnium, sodium, and barium in a reaction zone possessing a temperature gradient. The Bachelard et al. reaction requires specialized heating zones as well as elevated temperatures of up to 1500.degree. C.
Others have disclosed a carbothermal reaction between silica (SiO.sub.2) and carbon in the presence of pure nitrogen (N.sub.2) gas in an attempt to reduce the cost of producing silicon nitride as well as increasing its quality. Such attempts are reported in Durham et al., "Carbothermal Synthesis of Silicon Nitride: Effective Reaction Conditions," J. Am. Ceram. Soc., Vol. 74 (1), pp. 31-37 (1991). Durham et al. report that oxygen must carefully be removed from the reaction chamber, an excess of carbon relative to silica must be employed, and formation of silicon nitride only occurs within a very narrow temperature range of about 1350.degree. C. to 1550.degree. C.
A variety of other synthetic routes to silicon nitride are disclosed in U.S. Pat. Nos. 4,935,214 to Puger et al.; 5,232,677 to Fukuoku et al.; 5,258,169 to Wannagat et al; and 5,332,697 to Smith et al.
In view of the foregoing, it would be a significant advancement in the art to provide methods and systems for the synthesis of silicon nitride that avoided the use of dangerous and expensive chemical precursors.
It would be a her advancement in the art to provide methods and systems for manufacturing silicon nitride that could be carried out without dangerous and expensive chemicals while operating at a temperature far lower than presently required using conventional carbothermal methods.
It would be an additional advancement in the art if such methods and systems for low temperature synthesis of silicon nitride were able to employ catalytic means in order to carry out a modified carbothermal reaction sequence for silicon nitride at lower temperatures.
It would be a considerable advancement in the art to provide methods and systems for manufacturing silicon nitride that could utilize commonly found materials which were inexpensive or even considered to be waste products.
Such methods and systems for manufacturing silicon nitride are disclosed and claimed herein.